Most gages are accurate as delivered from the manufacturer, but every gage embraces certain limitations and assumptions. When selecting a gage or a gaging method, it is essential to establish a clear objective.

We have introduced the subject of circular geometry gaging by looking at the instrumentation, and we noted that one reason for the recent proliferation of geometry gages is the use of personal computers as gage controllers.

With the implementation of touch screen operation, canned measurement routines and automatic ranging, form measuring machines have become nearly foolproof and have successfully migrated from the lab to the shop floor.

Last issue we looked at some of the setup and analysis issues that can influence the results of your form measurement, including: the way the parts are staged; the influence of the stylus tip; using the wrong filter; confusion over the look of the graph; measuring the wrong parameter; and the influence of poor part tilt/centering.

In previous columns we discussed the relationship between part geometry (i.e., roundness) and dimensional tolerance. Circular geometry gages, with their precision spindles, are the best and the standard method for measuring out-of-roundness.

Form or geometry gages are generally divided into two categories depending on what form parameters they are designed to measure. While there is no formal designation to differentiate these two categories, sometimes they are simply referred to as roundness gages or cylindricity gages. Their difference is primarily based on their ability to make self-referenced or datum-referenced measurements.